The long-term objectives of this project are to identify and characterize pathogenic anticardiolipin antibodies (aCL) in the antiphospholipid syndrome (APS). Recently, we found 4/5 patient-derived, prothrombotic, monoclonal IgG aCL bind to plasmin, thrombin and activated protein C (APC). Of these 4 plasmin-reactive aCL, CL15 inhibits plasmin-mediated fibrinolysis and anticoagulant activity of APC, and causes fetal loss when injected into pregnant mice. It binds to plasmin with a relative Kd value of 7 x 10/-10 that is 3-4 logs higher than the affinity of IgG anti-beta2GPI antibodies (Ab) toward beta2GPI, the major autoantigen in APS. These data lead us to hypothesize that plasmin is an important autoantigen that drives certain IgG aCL in some APS patients, and that some of the plasmin-driven IgG aCL are prothrombotic (like CL15). To test these hypotheses, the specific aims are: 1. To study the prevalence, characteristics and pathogenic significance of the plasmin-reactive IgG aCL in APS patients. IgG will be purified and analyzed for Ab against CL and plasmin, and by cross inhibition to determine the presence of plasmin-reactive IgG aCL. The plasmin-reactive IgG aCL will be affinity purified and analyzed for binding affinity to plasmin, reactivity with thrombin an APC, and for lupus anticoagulant activity and pathogenicity. 2. To immunize mice with human plasmin and study the induced Ab and the pregnancy outcomes. Blood samples will be analyzed for IgG Ab against human plasmin and CL, and by cross inhibition to determine the presence of plasmin-reactive IgG aCL. Female mice with high titers of plasmin-reactive IgG aCL will be mated and the pregnancy outcomes will be examined to assess the pathogenicity of the plasmin-reactive IgG aCL. 3. To generate and study monoclonal plasmin-reactive aCL from immunized mice. The monoclonal Ab will be injected (individually or in pairs) into pregnant mice to study their pathogenic potentials, and be analyzed for reactivity with murine plasmin, and human thrombin and APC, and for relevant functional activity.